The present invention relates to a repetition pipette as defined in the introductory portion of claim 1.
A repetition pipette of this type has become known from German patent 29 26 691. In this pipette the syringe receiving section is a U-shaped groove in a pipette housing into which a syringe flange can be inserted through a lateral housing opening. In the receiving section the syringe flange is biased by a compression spring towards the delivery opening. The syringe piston includes a cylindrical actuating portion adapted to be fixed in the receiving member by clamping means. The piston returning means comprises a lever of the receiving member which extends through a lateral housing slot. The piston can be moved out of the syringe by movements of the lever away from the syringe flange. The piston advancing means comprises rack pawl means, the rack being connected to the receiving member and the pawl being pivotally mounted to a reciprocable drive lever. When the drive lever is pivoted towards the syringe flange, the pawl is catched so as to drive the rack and the piston connected therewith in the same direction. When the pawl is pivoted in the opposite direction, the pawl disengages from the saw-tooth-shaped toothing so that the piston does not change its position. The step width adjusting means comprises a tongue coupled to a rotary knob which covers the toothing more or less depending on the position of the rotary knob. Adjusting the rotary knob allows to adjust also that portion of pawl movements along which the pawl engages the tooth so as to drive the piston. The volume of liquid dispensed by the syringe is adjustable by means of the rotary knob.
The syringe is received from its receiving means with a certain play. Furthermore the adjusting means operate with play. As a result the piston is moved in a first advancing step for another distance than in the following steps. Accordingly, the liquid volume dispensed at the first step differs substantially from the liquid volume dispensed in any following step. In practice the liquid dispensed in the first step is thrown away to provide for precise dosing. As a result probe liquid is lost. This may be overcome by providing for relatively small first step width and by adjusting the width to the required values for the succeeding steps. However, this requires cumbersome handling and may result in errors.